I fitted a second horn to my 2011 Plus4. The necessary wiring was tied close to the existing horn.
The horns are normally installed as a combination of hi tone and Lo tone.
So check which one you have installed and buy the other.
The type hi or Lo is stamped on the horn allied the voltage info etc.

I bought the same make and model as originally fitted by the factory, from Holden Vintage and Classic car parts. About 9.50 plus vat.
They are "Plastic windtone" horns
Low tone 080.468
high tone 080.469

Very easy to fit. This setup does not require a separate relay.


Ian
2011 Plus 4
Wild Mogs Reims 2019, Goodwood 2018, Isle of Man 2017, Carried on to Caramulo" 2016